1. Field of the Invention
This invention relates to apparatus providing a continuous energy or fluid transmission between a stationary and a continuously rotating element.
2. Prior Art
There are numerous devices and machines requiring a continuous energy or fluid flow communication between elements which are continuously rotating relative to one another in one and the same direction. For example, an electric current or a stream of gas or liquid may have to be transmitted between a stationary element and a continuously rotating element. It is common practice in such instances to use coupling devices of the slip-ring or rotary-seal type, i.e. devices having interengaging relatively rotating parts. Such devices, however, often cause problems. Thus, in the case of electric slip-ring coupling devices noise, varying contact resistance and wear may cause problems, and in the case of rotary seals the main problem is the difficulty of providing a reliable fluid-tight seal between the relatively rotating parts.
It is possible, however, to transmit through a flexible electrical cable, flexible tube or other continuous, flexible filiform transmission element, an electric current or a stream of liquid or gas from a stationary, non-rotating terminal to a continuously rotating terminal without using a coupling device of the slip-ring or rotary-seal type. Thus, it is feasible to fasten one end of the filiform transmission element to the stationary terminal and fasten the other end of the transmission element to the rotating terminal and to effect the transmission through the transmission element without twisting the latter.
In a prior art device permitting such transmission, a filiform transmission element in the form of a flexible tube or a flexible electric cable has one end vertical and immovably secured to a stationary part while its other end is horizontal and rotatably supported on a main rotor. The axis of rotation of the main rotor is vertical and passes through the first end of the transmission element. As the main rotor, and hence the right-angle bight or bend formed by the transmission element, rotate about the vertical axis, the horizontal end of the transmission element rotates about its own horizontal axis at the same rotational speed as the main rotor, provided that no twisting of the transmission element takes place. The horizontal end of the transmission element is fastened to a secondary rotor which is supported on the main rotor and rotatable about the longitudinal axis of the horizontal end of the transmission element. Using this arrangement, an electric current or a stream of gas or liquid can be continuously transmitted from the immovably secured end into the continuously rotating secondary rotor. Examples of such arrangements are shown in U.S. Pat. Nos. 3,657,941 and 3,856,669.
The above-described prior art device may be modified by further bending the transmission element such that its ends point in the same direction and mounting the secondary rotor for rotation about an axis which is substantially parallel to, and preferably coincident with, the axis of rotation of the main rotor. The bend or bight formed by the transmission element thus resembles a fishhook or an interrogation mark. If this bight, which is journalled in the main rotor for rotation about its own curved longitudinal axis relative to the main rotor, is caused to revolve about the axis of rotation of the main rotor and the secondary rotor is caused to rotate in the same direction but at twice the rotational speed at which the bight revolves, the rotational movements cause no twisting of the transmission element. A device modified in the above-described manner is shown in U.S. Pat. No. 3,586,413.
The modified device may be embodied e.g. as a centrifuge for the continuous separation of blood into two fractions, a plasma fraction and a blood cell fraction. A known centrifuge of this type (U.S. Pat. No. 4,056,224) comprises two rotors, namely, an outer rotor (the main rotor) which is mounted in a base and rotates relative to the base about a common vertical axis, and an inner rotor (the secondary rotor), which is rotatably mounted in the outer rotor and caused to rotate relative to it about the common vertical axis. A drive system comprising a motor and a transmission system interconnecting the two rotors rotates the rotors in the same direction relative to the base, the inner rotor rotating at twice the speed of the outer rotor.
The inner rotor carries a separation container to which a filiform transmission element formed by a bundle of flexible tubes is fastened. The bundle of tubes extends downwardly from the inner rotor or the separation container at a point on or near the vertical axis of rotation and then radially outwardly and upwardly around the region occupied by the inner rotor and then extends radially inwardly back to the vertical axis of rotation, where the bundle is fastened to the base by a clamp or other holder. The tip of the bight formed by the bundle of tubes, which resembles a fishhook or an inverted interrogation mark, is adjacent the separation container on the inner rotor while the shank extends through the clamp or holder on the base. This bight revolves around the vertical axis as the outer rotor rotates.
Blood is continuously fed into the separation container through one of the flexible tubes, and one or both of the plasma and blood cell fractions formed in the separation container are continuously withdrawn through the other tubes. The separation thus is effected continuously without any contacting of the blood or the fractions with relatively rotating parts during the passage between the continuously rotating separation container and the portion of the tubes fastened to the base. It is easy, therefore, to maintain the separation system completely closed, i.e. to completely isolate the blood and the blood fractions from the surrounding environment. This, in turn, means that the separation may be carried out in sterile conditions. Moreover, since the blood or the blood fractions are not contacted by any relatively rotating parts, mechanical damage on the blood cells is avoided.
The individual tubes or the bundle of tubes need not be capable of accommodating any appreciable degree of twisting as long as the inner rotor rotates at exactly twice the speed of the outer rotor. In the known centrifuge the exact 2:1 speed ratio is ensured by rotationally interconnecting the rotors by means of a gear or gear belt transmission. Since the bundle of tubes has to rotate about its own curvilinear, fishhook-shaped longitudinal axis relative to the outer rotor, namely at a speed equal to the speed at which the outer rotor rotates relative to the base, the friction between the bundle of tubes and the outer rotor gives rise to certain problems. In addition, the presence of air bubbles in the tubes may result in blocking of the flow through the tubes. These problems are caused by the centrifugal force which presses the tubes against the surfaces on the outer rotor retaining or guiding them and of course also acts on the liquid in the tubes; in accordance with well-known laws of physics the magnitude of the centrifugal force is proportional both to the distance from the axis of rotation and the square of the speed at which the bight formed by the tubes revolves around the axis of rotation, i.e. the square of the speed of the outer rotor.
Consequently, even a relatively small reduction of the speed of the outer rotor would result in a substantial reduction of the friction between the tubes and the outer rotor. On account of the given speed ratio, this would result in a substantial reduction of the speed of the inner rotor and hence of the centrifugal force on the blood in the separation vessel.